Conventionally, a lithographic offset printing method, a letterpress printing method, a letterpress offset printing method, or a screen printing method is carried out as a multiple color printing method for thin metals such as metallic plates or metal foils, or metallic containers. The lithographic offset printing method is a method of the type in which an ink is applied to a picture-line portion of a lithograph provided with a picture-line portion of lipophilic property and a non-picture-line portion of hydrophilic property, the ink on the lithograph is transferred to a rubber blanket, and the ink on the rubber blanket is then printed on a material to be printed. The letterpress printing method is of the type in which an ink is applied to a picture-line portion in form of relief on a press and the ink is then printed on a material to be printed. The letterpress offset printing method is called in a sence a dry-offset printing method of the type in which an ink is applied to a picture-line portion of a letter press provided with a protruded picture-line portion and a recessed picture-line portion, the ink on the letterpress is transferred to a rubber blanket, and the ink on the rubber blanket is printed on a material to be printed. The screen printing method is of the type in which an ink is printed on a material to be printed by using a screen press provided with a picture-line portion only through which the ink is permeated. These conventional printing methods are superior in the mass production of the printed materials, but require the plates and the plate-making process beforehand the printing requires much time and labour. Recently, an electronic technique has been developed in the field of the printing technique, for example, computerization utilizing a layout scanner in an original manufacturing stage and development of a direct plate-making system in a plate-making process. However, no technique for eliminating the plate-making process has been yet developed, and accordingly, the defects described above have not been yet solved.
In addition, in accordance with the variety of the value judgements, there is an increasing requirement of the printing of the small amount of the multiple kinds of products and it becomes difficult to satisfy this requirement by the conventional methods which lack in an instantaneous printing function.
In the meantime, as a printing technique utilizing no printing press, are known an electrophotographic printing method, an ink-jetting methoc, or a thermo-transferring method, which are so called no-inpact printing technique in a case where paper is used as a material to be printed. According to these methods, a picture image can be directly obtained by a picture image output of a computer without using a printing press. Particularly, the electrophotographic printing method is utilized for a copying machine, facsimile, or printer and is watched as a method to be substituted for the conventional printing methods. As an application of the electrophotographic printing method to the printing technique, there is disclosed an apparatus for manufacturing a multiple color label in the Japanese Patent Laid-open Publication No. 23355/19874. The described invention is characterized by the multiple color label manufacturing apparatus wherein a photoconductive material is charged by charging devices in a plurality of electrophotographic printing devices, charged latent images are formed on the surface of the photosensitive material by projecting light corresponding to the original by means of an exposure by device, the latent images are visualized by sticking toners on the surface of the photosensitive material by means of a developing device, the toner images are realized on the surface of a base material for the label by means of a transferring device, and finally, the thus visualized images are fixed by means of fixing device. However, a multiple color printing technique for the metallic containers or thin plate metals in application of this electrophotographic printing technique has not been yet practically utilized.
An object of this invention is to improve the defects or drawbacks described above and to provide a multiple color printing method for a metallic container or thin metal plate capable of press-less printing utilizing no press.
Another object of this invention is to provide a multiple color printing method for a metallic container of thin metal plate capable of printing the small amount of multiple kinds of products by instantaneously printing picture image information of an original stored in the computer.
A further object of this invention is to provide a multiple color printing method capable of clearly or finely carrying out the printing on an extremely thin flat metal plate or a metallic container having a curved surface on which the printing is effected.
A still further object of this invention is to provide a multiple color printing method for a metallic container or a thin metal plate capable of including no crushed image due to the printing pressure which may be observed in the lithographic offset printing or a gravure printing.
By the way, in the application of the described electrophotographic printing technique to the multiple printing method for the metallic material, there is a problem on the fixing process.
When the paper is used as a material to be printed, the heat conductivity of the paper is small, so that the heating only of the surface thereof can be done, resulting in reduced energy consumption, and moreover, the cooling can be speedily made, thus being capable of performing high speed multiple color printing. On the other hand, when the metallic material is used as a material to be printed, since the heat conductivity thereof is large, it is difficult to heat only the surface thereof and the metallic material is entirely heated, resulting in the large consumption of heating energy. Moreover, in the multiple color printing, it is necessary to cool the material to be printed to a room temperature every color printing operation, thus being difficult to print the material at a high speed.
Accordingly, a still further object of this invention is to improve the problem described above and to provide a multiple color printing method of a thin metallic material capable of printing the same at a high printing speed with reduced energy consumption.
Furthermore, in the application of the electrophotographic printing technology to the multiple color printing method for a metallic container or thin metal plate, there is a problem on the transferring process. Namely, since the photoconductive layer and the metal have hard surfaces, the smooth registration between the metal and the toner cannot be obtained to carry out the clear transfer even if these surfaces are pressed with each other with the toner interposed.
With the metallic container, particularly a metallic can, since a spray coating or finish baking is applied to the inner surface thereof after the printing process, it is required for the toner to have a heat resisting property, and in the subsequent neck-in working or flange forming working, the flexibility and adhesive property are also required. In addition, in the steam sterilizing process at a temperature more than 100.degree. C. after filling the content in the thus treated metallic can, it is also required for the toner to have a water-proof and heat-resisting properties. A printed material satisfied with the above requirements was not obtained by carrying out the heat fixing operation only to the toner by the electrophotographic printing method. This invention is conceived to solve this problem and aims to provide a multiple color printing method for a metallic container of thin metal plate capable of obtaining a strong printed material having an improved flexibility and adhesive property with no crushed picture image due to the printing pressure.